Adenosine undergoes rapid metabolic degradation after the intracerebral injection into the rat brain

The study was aimed to elucidate the metabolic fate of [14C]adenosine after the injection into the brain and was performed on six Wistar rats (250 g).

The rats were anaesthetised (thiopenthone-Na, 40mg kg-1I.P.), placed in a stereotaxic frame, a hole drilled in the skull, and the tip of a needle lowered into the left hemisphere to the point known as Par2 (Kakee et al. 1996). That needle was connected to micro syringe (Hamilton) and 0.5ml of artificial interstitial fluid containing 0.2 mCi of [14C]adenosine was injected into the brain slowly over 2 s and then 0.5 ml of blood collected from the left jugular vein carefully between the 3rd and 5th minute after the intracerebral injection. These samples were centrifuged, plasma samples were boiled for 5 min and centrifuged again to precipitated proteins, and 20 ml of supernatant of each sample was used for liquid chromatography (HPLC) separation of purines, which was followed by simultaneous scintillation counting of radioactivity. The result revealed that (mean ± S.E.M.) 80.43 ± 11.23% and 7.11 ± 3.61% of total radioactivity in plasma appeared within peaks of hypoxanthine and adenine (n = 3), respectively, the two nucleobases that could be considered as the metabolic degradation products of adenosine. Only the background radioactivity could be detected within the adenosine spot. When 0.5 mM erythro-9-(2-hydroxy-3-nonyl)-adenosine, a potent inhibitor of adenosine deaminase, was injected into the brain simultaneously with [14C]adenosine, 38.45±6.93% of total radioactivity in the plasma appeared within the adenosine peak (n = 3). Therefore, it seems that the blood-brain barrier represents a powerful enzymatic barrier for adenosine, and that the adenosine deaminase plays a central role in that function.